Alterations in apolipoprotein gene transcription may cause changes in plasma lipid and lipoprotein levels and in some instances may increase the risk of cardiovascular disease. The long-term objective of the proposed studies is to understand the mechanisms of transcriptional regulation of the human apolipoprotein genes in vivo. The rationale of our approach and our hypothesis is that if we understand the molecular events and the signal transduction pathway(s) which lead to gene activation, then we may be able to selectively regulate apolipoprotein and lipoprotein synthesis and thus regulate plasma lipids and lipoprotein levels for optimal health. As a model system for understanding gene regulation in cell cultures and in experimental animals, we will utilize the human apoA-I/apoCIII gene cluster. Key observations during the last five years have established that the -800/-590 apoCIII regulatory region is an intestinal enhancer and that nuclear receptors and SP1 play an important role in the regulation of the apoA-I and apoCIII genes. Our specific aims are: 1) To elucidate how activation of cJun and ATF-2 and HNF-4 cascades, via specific signal transduction, affect the expression of the human apoCIII gene in cell cultures. 2) To elucidate what combinations of promoter and enhancer elements determine the tissue-specific expression of the apoA-I and apoCIII genes in vivo using transgenic and knock-in mouse models. 3) To elucidate the contribution of different nuclear receptors as well as the transcription factors such as SP1 and C/EBP to the hepatic and intestinal expression of the human apoA- I and apoCIII genes in vivo using existing animal models, antisense methodologies and adenovirus mediated gene transfer. It is expected that the proposed studies will provide new insights into the mechanism of transcriptional regulation of the apoA-I and apo-CIII genes as well as general insights into hepatic and intestinal gene regulation. Increases in plasma apoA-I and HDL levels are associated with protection from cardiovascular disease. Alteration in apoCIII has been shown to affect the catabolism of triglyceride-rich lipoproteins. Thus the information obtained from this project may provide rational approaches towards correcting low plasma HDL levels and reducing hypertriglyceridemia in humans.